The present invention is directed to compositions and processes for the development of electrostatic latent images. More specifically, the present invention is directed to compositions and processes for developing electrostatic latent images with liquid developers having a vehicle that is in liquid form under development conditions and in solid form subsequent to development. One embodiment of the present invention is directed to a process for forming images which comprises (a) generating an electrostatic latent image, (b) contacting the latent image with a developer comprising a colorant and a substantial amount of a vehicle with a melting point of at least about 25.degree. C., said developer having a melting point of at least about 25.degree. C., said contact occurring while the developer is maintained at a temperature at or above its melting point, said developer having a viscosity of no more than about 500 centipoise and a resistivity of no less than about 10.sup.8 ohm-cm at the temperature maintained while the developer is in contact with the latent image, and (c) cooling the developed image to a temperature below its melting point subsequent to development.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. For example, U.S. Pat. No. 2,297,691 discloses an electrophotographic imaging process that entails placing a uniform electrostatic charge on a photoconductive insulating layer, such as a photoconductor or photoreceptor, exposing the photoreceptor to a light and shadow image to dissipate the charge on the areas of the photoreceptor exposed to the light, and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic material known as toner. When the toner is charged to a polarity opposite to that of the latent electrostatic image on the photoreceptor, the toner will normally be attracted to those areas of the photoreceptor which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. When the toner is charged to the same polarity as that of the charge applied to the photoreceptor, the toner will normally be attracted to those areas which have been discharged; this process is known as discharge area development. This developed image may then be transferred to a substrate such as paper and subsequently be permanently affixed to the substrate.
In ionographic imaging processes, a latent image is formed on a dielectric image receptor or electroreceptor by ion deposition, as described, for example, in U.S. Pat. Nos. 3,564,556, 3,611,419, 4,240,084, 4,569,584, 2,919,171, 4,524,371, 4,619,515, 4,463,363, 4,254,424, 4,538,163, 4,409,604, 4,408,214, 4,365,549, 4,267,556, 4,160,257, and 4,155,093, the disclosures of each of which are totally incorporated herein by reference. Generally, the process entails application of charge in an image pattern with an ionographic writing head to a dielectric receiver that retains the charged image. The image is subsequently developed with a developer capable of developing charge images.
For electrophotographic and ionographic processes, either dry or liquid developers may be used for development of the electrostatic latent image. Liquid electrophoretic developers generally comprise a liquid vehicle in which is dispersed charged colored toner particles. In liquid electrophotographic development processes, the photoreceptor bearing the electrostatic latent image is transported through a bath of the liquid developer. Contact with the charged areas of the photoreceptor causes the charged toner particles present in the liquid vehicle to migrate through the liquid to the charged areas of the photoreceptor to develop the latent image. Thereafter, the photoreceptor is withdrawn from the liquid developer bath with the charged pigment particles adhering to the electrostatic latent image in image configuration. If desired, the image may be treated to remove some of the liquid vehicle. The developed image is then transferred to a suitable substrate, such as paper or transparency material, and, optionally, may be fixed to the substrate by heat, pressure, a combination of heat and pressure, or other suitable fixing means such as solvent or overcoating treatment. A similar process is used to develop latent images formed on ionographic imaging members.
In addition, liquid electrophoretic development of electrostatic latent images on charged papers is known. In these processes, the electrostatic latent image, which is usually formulated on a single sheet of dielectric paper, is transported through a bath of the liquid developer. Contact with the liquid developer causes the charged toner particles present in the liquid developer to migrate through the liquid vehicle to the dielectric paper in the configuration of the latent image. Thereafter, the sheet is withdrawn from the liquid developer bath with the charged toner particles adhering to the electrostatic latent image in image configuration. The thin film of residual developer remaining on the surface of the sheet is then evaporated within a relatively short time period, usually less than 5 seconds. Subsequently, the marking pigment particles may optionally be fixed to the sheet by any suitable method.
Polarizable liquid developers can also be used to develop electrostatic latent images. In polarizable liquid development processes, as disclosed in U.S. Pat. No. 3,084,043, the disclosure of which is totally incorporated herein by reference, liquid developers having relatively low viscosity and low volatility and relatively high electrical conductivity (relatively low volume resistivity) are deposited on a gravure roller to fill the depressions in the roller surface. Excess developer is removed from the lands between the depressions, and as a receiving surface charged in image configuration passes near the gravure roller, liquid developer is attracted from the depressions onto the receiving surface in image configuration by the charged image. Developers and processes of this type are disclosed in, for example, U.S. Pat. Nos. 4,047,943, 4,059,444, 4,822,710, 4,804,601, 4,766,049, Canadian Patent 937,823, Canadian Patent 926,182, Canadian Patent 942,554, British Patent 1,321,286, and British Patent 1,312,844, the disclosures of each of which are totally incorporated herein by reference.
In photoelectrophoretic liquid development processes, as disclosed in, for example, U.S. Pat. Nos. 4,135,925, 3,383,993, 3,384,488, 3,384,565, 3,384,566, 4,043,655, and 4,023,968, the disclosures of each of which are totally incorporated herein by reference, colored photosensitive toner particles are suspended in an insulating carrier liquid. The suspension is placed between at least two electrodes subjected to a potential difference and exposed to a light image. Typically, the imaging suspension is placed on a transparent electrically conductive support in the form of a thin film and exposure is made through the transparent support while a second biased electrode is rolled across the suspension. It is believed that the particles bear an initial charge once suspended in the liquid carrier which causes them to be attracted to the transparent base electrode upon application of the potential difference. Upon exposure, the particles change polarity by exchanging charge with the base electrode so that the exposed particles migrate to the second or roller electrode, thereby forming the images. Both polychromatic and monochromatic images can be formed by the process; when polychromatic images are prepared, the liquid developer can contain toner particles of more than one color.
Typically, liquid developers employ aliphatic saturated hydrocarbons as liquid vehicles, most commonly high boiling aliphatic hydrocarbons that are relatively high in resistivity and nontoxic. Developers and processes of this type are disclosed in, for example, U.S. Pat. Nos. 4,476,210, 2,877,133, 2,890,174, 2,899,335, 2,892,709, 2,913,353, 3,729,419, 3,841,893, 3,968,044, 4,794,651, 4,762,764, 4,830,945, 4,686,936, 4,766,049, 4,707,429, 4,780,388, 3,976,808, 4,877,698, 4,880,720, 4,880,432, and copending application U.S. Ser. No. 07/300,395, the disclosures of each of which are totally incorporated herein by reference.
U.S. Pat. No. 4,659,640 (Santilli) discloses a liquid electrographic developer containing a volatile, electrically insulating carrier liquid, polyester toner particles, and wax dispersed in the carrier. The wax-to-polyester weight ratio in the developer is sufficiently high to render the developer self-fixing at room temperature. Upon application of the developer to a latent image and evaporation of the liquid carrier from the image, the toner, aided by the wax at the indicated concentration level, is fixed to the surface without the need for externally applied heat.
U.S. Pat. No. 4,557,991 (Takagiwa et al.) discloses a toner for development of electrostatic images which comprises a wax comprising a polyolefin and a binder resin selected from the group consisting of a polyester resin, a vinyl polymer, a styrene-butadiene copolymer, a styrene polymer, a styrene-containing copolymer, and a polymer containing a reactive prepolymer.
U.S. Pat. No. 4,842,974 (Landa et al.) discloses a liquid composition for developing latent electrostatic images comprising toner particles associated with a pigment dispersed in a nonpolar liquid. The toner particles are formed with a plurality of fibers of tendrils from a thermoplastic polymer and carry a charge of a polarity opposite to the polarity of the latent electrostatic image. The polymer is insoluble or insolvatable in the dispersant liquid at room temperature. The toner particles are formed by plasticizing the polymer and pigment at elevated temperature and then either permitting a sponge to form and wet-grinding pieces of the sponge or diluting the plasticized polymer-pigment while cooling and constantaly stirring to prevent the forming of a sponge while cooling.
U.S. Pat. No. 4,130,670 (Gilliams et al.) discloses a sheet or web material for use in developing and fixing toner images which comprises a support and a thermoadhesive fixing layer defining the surface of the material on which the toner image is deposited. The thermoadhesive fixing layer comprises an organic polymeric material and has a surface resistance above 10.sup.10 ohm/square, freedom from blocking at least up to 35.degree. C., a melt viscosity at 190.degree. C. of not more than 120 P, and an abrasion resistance at 20.degree. C. above 175 g. The process for fixing a toner image on this sheet or web material comprises the steps of image-wise depositing on the thermoadhesive fixing layer toner particles forming a contact angle with the molten thermoadhesive fixing layer smaller than 90.degree., heating above 90.degree. C. at least those parts of the layer corresponding with the toner images, said heating being of a sufficient intensity and duration that the particles sink within the softened fixing layer, and allowing the imaged layer to cool to fix the image particles in the layer.
U.S. Pat. No. 4,137,340 (Verlinden et al.) discloses a method for fixing a liquid toner image on a thermoadhesive layer of a recording material by irradiating the layer with high intensity short duration light pulses. The energy is sufficiently high to cause a partial melting of the layer so that the toner particles become absorbed thereon but the duration of the light pulse is sufficiently short to avoid a permanent deformation of the recording material itself.
Copending application U.S. Ser. No. 07/654,693, filed Feb. 13, 1991, entitled "Curable Liquid Developers With Heterogeneous Initiators", with the named inventors Ian D. Morrison, Bing R. Hsieh, and Jerry H. Taylor, the disclosure of which is totally incorporated herein by reference, discloses a liquid developer comprising a colorant and a substantial amount of a curable liquid vehicle having a viscosity of no more than about 500 centipoise and a resistivity of no less than about 10.sup.8 ohm-cm. One embodiment of the invention is directed to an electrophoretic liquid developer comprising a substantial amount of a curable liquid vehicle having a viscosity of no more than about 20 centipoise and a resistivity of no less than about 5.times.10.sup.9 ohm-cm, a charge control agent, and colored particles capable of becoming charged and migrating through the liquid vehicle to develop an electrostatic latent image. Another embodiment of the invention is directed to a polarizable liquid developer comprising a colorant and a substantial amount of a curable liquid vehicle having a viscosity of from about 25 to about 500 centipoise and a resistivity of from about 10.sup.8 to about 10.sup.11 ohm-cm. Yet another embodiment of the invention is directed to a photoelectrophoretic liquid developer comprising a substantial amount of a curable liquid vehicle having a viscosity of no more than about 20 centipoise and a resistivity of no less than about 5.times.10.sup.9 ohm-cm and photosensitive colored particles. A specific embodiment of the invention is directed to a liquid developer comprising a colorant, a substantial amount of a curable liquid vehicle having a viscosity of no more than about 500 centipoise and a resistivity of no less than about 10.sup.8 ohm-cm, and solid particles containing an initiator substantially insoluble in the liquid vehicle and capable, upon activation, of initiating polymerization of the curable liquid vehicle. In one embodiment, the colorant comprises pigment particles and the initiator is contained on the surfaces of the pigment particles. In another embodiment, the developer contains polymeric particles and the initiator is contained on the surfaces of the polymeric particles. In yet another embodiment, the colorant comprises toner particles which comprise a pigment and a polymer, and the initiator is contained on the surfaces of the toner particles. In still another embodiment, the initiator is contained on the surfaces of solid particles such as silicas, clays, or the like. The initiator can also be contained within the solid particles.
While known compositions and processes are suitable for their intended purposes, a need remains for liquid development compositions and processes that produce prints with little or substantially no odor. A need also remains for liquid development compositions and process that reduce or substantially eliminate the emission or carryout of solvent vapors from copiers and printers. Further, there is a need for liquid development compositions and processes that reduce or eliminate the need to dispose of solvents from a copier or printer employing liquid development. Additionally, there is a need for liquid development compositions and processes that enable the formation of high quality images on a wide variety of substrates. There is also a need for liquid development compositions and processes that enable easy handling of the developer material in that a solid material, as opposed to a liquid material, is used to replenish the developer. Further, there is a need for liquid development compositions and processes that enable easy disposal of the developer material in that a solid material, as opposed to a liquid material, is discarded from the machine. Additionally, there is a need for liquid development compositions and processes that reduce or eliminate the carryout of liquids from the imaging apparatus onto the final substrate or any intermediate transfer elements employed. There is also a need for liquid development compositions and processes that enable enhanced flexibility in the design of the imaging apparatus, particularly with respect to the location of the process elements around the imaging member